Oblivion:Essential NPCs

This page contains a complete list of all NPCs who at some point during gameplay are "essential." What it means is that they cannot be killed - if you try, they'll merely be knocked unconscious. You can tell if an NPC is essential in-game, because their talk-icon will be a crown, instead of the standard talk icon. Most of these NPCs become killable later in the game.

It should be noted that, while essential NPCs are not supposed to be killed, it is still possible to kill them by shoving them into an Abyss (this can potentially happen with follower NPCs, since they cannot simply jump over the Abyss like you can). This can cause many bugs in the game that being essential was supposed to prevent.